The present embodiments relate to motion artifacts in ultrasound imaging. Ultrasound transducers may recoil due to transmission of acoustic energy, causing a motion artifact.
Recoil may occur in any mode of ultrasound imaging. Shear wave and elasticity imaging modes may be more susceptible to motion artifact from recoil in modes such as acoustic radiation force impulse (ARFI) imaging due to the greater acoustic energy used in push pulses. ARFI provides the ability to assess the stiffness of tissue, both in a relative and absolute sense. ARFI imaging exploits the fact that a portion of the acoustic energy transmitted into the tissue is converted into a radiation force that acts on the tissue in the direction of the acoustic beam. This force is proportional to the attenuation and the spatially averaged intensity of the acoustic pulse. The stiffness properties of the tissue can be ascertained by interrogating the induced tissue displacements in the longitudinal or transverse (shear) directions.
ARFI imaging may be performed with a transducer array mounted on a catheter, such as a cardiac catheter. One consequence of ARFI imaging that impacts flexible intra-cardiac echo (ICE) devices is that a long high intensity acoustic pulse transmitted from the catheter generates a reactionary force or recoil. The recoil may generate appreciable displacement in the transducer itself. Hsu, et al., “Challenges and Implementation of Radiation Force Imaging with an Intracardiac Ultrasound Transducer,” IEEE UFFC, Vol. 54, no. 5, May, 2007, have demonstrated that the motion in an ICE device peaks at about 8 um after 5 msec (see FIG. 1a, reproduced from the Hsu article). The average maximum longitudinal tissue displacements that are observed in ARFI imaging are on the order of 2-6 um, as shown in Table 1, reproduced from the Hsu article.
TABLE 1PRE-AND POSTABLATION AVERACEMAXIMUM DISPLACEMENTS.Average maximum displacement (μm)LeftCenterRight1Preablation2.63 ± 0.335.60 ± 1.104.72 ± 1.52Mid-ablation 2.59 ± 0.324.91 ± 0.673.45 ± 0.78Postablation2.70 ± 0.255.31 ± 1.132.46 ± 0.44The motion of the transducer (e.g., 8 um) may completely confound the estimation of the ARFI induced tissue displacements (e.g., average 2-6 um).
To address this problem, Hsu, et al. take a brute force approach. A long series of acoustic pulses are transmitted to push the transducer head back to a pre-loaded position before beginning ARFI imaging. The imaging pulses that are fired to create a 2D ARFI image then cause a roughly linear increase in displacement with each repetition of a pushing or ARFI pulse. FIG. 1b, reproduced from the Hsu article, shows about 25 ms worth of preloaded transmissions to place the transducer in a position to respond to ARFI with linear offset. The preloading transitions the transducer through nonlinear movement and into a linear movement region starting at about 30 ms. However, the ability to transmit such long preloading pulses may be limited by heating and power considerations.